Foamed thermoplastic resin comprising poly(p-methylenebenzoate)

ABSTRACT

Foamed thermoplastic resin comprising poly(p-methylenebenzoate) suitable for structural foam injection molded parts.

FIELD OF THE INVENTION

The field of this invention relates to the preparation of foamedthermoplastic resin comprising poly(p-methylenebenzoate).

BACKGROUND OF THE INVENTION

Foamed thermoplastic resin comprising poly(p-methylenebenzoate) has notbeen previously prepared. Although p-hydroxymethylbenzoic acid, which isthe monomer for poly(p-methylenebenzoate) has been known since as earlyas 1872 when the acid was prepared by free-radical bromination ofp-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueousbarium hydroxide and subsequent purification by recrystallization fromwater, the low molecular weight of the resulting polymer made thepolymer obtained from the acid unsuitable for use in foams. In general,it is known that molecular weight of a polymer must be sufficiently highto cause the resulting polymer to have good cell structure when foamed.Polymers having insufficient molecular weight, or insufficient meltstrength, have been demonstrated to not have sufficient cell strength toform bubbles properly, the bubbles tending to deform and tear apartcaused by lack of strength to withstand the inflation pressure of thebubble. Melt strength is correlated with measurement of inherentviscosity which, in turn, is used to measure molecular weight. Aninherent viscosity (I.V.) of the polymer, which is lower than 0.4 dl/g,as measured in deciliters per gram (dl/g) in a 60/40phenol/tetrachloroethane solvent at 30° C., is considered too low toallow proper cell formation of foamed poly(p-methylenebenzoate).

Polymers of p-hydroxymethylbenzoic acid having an inherent viscosity ofat least 0.44 dl/g have been taught in commonly-assigned U.S. Pat. No.4,130,719, but foamed thermoplastic resin comprisingpoly(p-methylenebenzoate) has not been previously prepared becausechoice of foaming agent had been found to be critical in preparing thefoamed composition. Commonly-used foaming agents such asazodicarbonamide have been found to be deleterious when used to formpoly(p-methylenebenzoate). Cell coalescence occurs because molecularweight of the polymer apparently decreases.

An object of this invention is to provide a foamed thermoplastic resinof poly(p-methylenebenzoate having regular structure, good mechanicalproperties and unique molding characteristics.

A further object of this invention is to provide a process for providinga foamed thermoplastic resin of poly(p-methylenebenzoate) using afoaming agent suitable for the process. The process of thermoplasticstructural foam injection molding is well known and adequately describedin many sources, notably, J. L. Throne, "Principles of ThermoplasticStructural Foam Molding: A Review," 77-131, in N. P. Suh and N. H. Sung,eds., Science and Technology of Polymer Processing, MIT Press, 1979,which is incorporated by reference, and will not be repeated in detailhere.

These and other objects will become apparent upon reading of thespecification.

SUMMARY OF THE INVENTION

Foamed thermoplastic resin comprising poly(p-methylenebenzoate) has highshear sensitivity, slow rate of crystallization and inherently highimpact resistance. The combination of properties makes the foamedpoly(p-methylenebenzoate) suitable for high-speed molding of thin-wallparts requiring relatively thick skins and high impact resistance. Theprocess of producing foamed poly(p-methylenebenzoate) requires an inertnon-toxic foaming agent such as an inert non-toxic gas or a chemicalfoaming agent such as 5-phenyltetrazole which decomposes to produce aninert non-toxic gas and chemically inert by-products.

DETAILS OF THE INVENTION

A method has now been discovered by which it is possible to producefoamed injection-molded thermoplastic resin products ofpoly(p-methylenebenzoate) as a homopolymer, and with blends, and withinterpolymers of poly(m-methylenebenzoate) andpoly(alkyleneterephthalate) polymers, and to do so with conventionalthermoplastic structural foam injection molding equipment operating atdesign capacity. The foams thus produced are characterized by uniformcell size in the foam core, substantial high density skins and highperformance-to-weight ratio. The foamed products are predominantlypoly(p-methylenebenzoate).

The foamed products can comprise poly(p-methylenebenzoate), andpoly(m-methylenebenzoate), and/or poly(alkyleneterephthalate) asphysical blends or mixtures of homopolymers ofpoly(p-methylenebenzoate), poly(m-methylenebenzoate) andpoly(alkyleneterephthalates) in varying proportions.

The foamed products can comprise poly(p-methylenebenzoate) orpoly(m-methylenebenzoate) with interpolymers ofpoly(p-methylenebenzoate), poly(m-methylenebenzoate) orpoly(alkyleneterephthalates) in varying proportions.

Interpolymers are defined as polymer compositions intermediate between aphysical blend and a totally random copolymer. Interpolymers occur dueto the presence of other polyesters than poly(p-methylenebenzoate) orpoly(m-methylenebenzoate) in the molding composition prior to themolding operation. The melt temperature of the molding operation, thepercentage of each component and the time duration of the melt in themolding operation often determine the structure of the interpolymer andwhether the polymer composition is that of a blend or of aninterpolymer.

In the process of the invention, 100 parts by weight of homopolymer, andblending polymers, are mixed with a controlled amount of foaming agent.Typically for chemical foaming agents, 0.25-1 parts by weight yieldadequate gas for foaming with the extreme limits being 0.01-10 parts byweight. The chemical foaming agent is typically mixed with the moldingcomposition at room temperature, prior to charging the mixture to thehopper or inlet section of the plasticating screw of the injectionmolding machine. The mixture is then elevated in temperature andpressure until the polymer melts and the chemical blowing agentdecomposes to produce gas. This mixture is then batchwise injected intoa region of reduced pressure whereupon the gas thus produced expands thepolymer to produce a cellular foam.

Further, 100 parts by weight of molding composition can be heated andmelted and a chemical foaming agent, such as 5-phenyltetrazole whichdecomposes to form a gas, or a physical foaming agent comprising aninert non-toxic gas, such as nitrogen gas, carbon dioxide, linearhydrocarbons of from 2 to 8 carbon atoms, such as butane, pentane,halogenated hydrocarbons of from 1 to 6 carbon atoms, such as methylchloride, ethyl chloride, or fluorocarbons such asdichlorodifluoromethane, 1,2-dichloro,1,1,2,2-tetrafluoroethane can bemetered into the melt, typically at 5 parts by weight but within thelimits of 0.01-50 parts by weight. The polymer melt-containing gas isthen batchwise injected into a region of reduced pressure whereupon thegas expands the polymer to produce a cellular foam. Foaming level can befrom about 0.001% to about 50%. A preferred range is from about 10% toabout 50%. A more preferred range is from about 10% to about 40%. A mostpreferred range is from about 15% to about 25%.

The term "foaming level" is a measure of the foam bulk density relativeto the bulk density of the unfoamed material. Foaming level is measuredin terms of reduced density of the material according to the following:##EQU1##

The preferred temperature of the resin-gas mixture just prior toinjection is 250°-300° C. at an injection pressure of 100-2000 psi,typically about 400-500 psi. When the temperature and pressureconditions are noticeably outside these ranges, certain foaming agentscan fail to perform satisfactorily. At temperature appreciably below250° C., the plastic may not melt and/or the particular chemical foamingagent selected may not fully decompose, thus yielding an unprocessiblesystem. If pressures fall below about 400 psi the gas may not bethoroughly dissolved in the polymer, thus causing very ragged cells tobe found in the final part.

The thermoplastic structural foam process, as currently practiced ininjection molding, is used to fabricate large, relatively thick-wallstructures. The process requires relatively long cooling times, on theorder of minutes. As a result, extant processing machinery melts andstores large quantities of polymer-gas mixture prior to each processinginjection into the mold cavity. Thus the polymer-gas mixture must remainthermally stable at processing temperatures (and pressures) for severalminutes. An informal stability criterion is time-temperature, obtainedby multiplying the melt temperature (°C.) by the residence time (sec) atthis temperature prior to injection. A typical operating range for thiscriterion is 5,000-30,000 (°C.-sec) with upper limit being 60,000(°C.-sec).

The term "homopolymer" as used in this specification and claims means apoly(p-methylenebenzoate) having an I.V. of at least 0.4 dl/g,preferably from 0.5 to 0.7 I.V. The term blending polymer means apoly(m-methylenebenzoate) and any poly(alkyleneterephthalate) of thestructural formula ##STR1## wherein n is a whole number from 2 to 20,wherein the ratio of poly(p-methylenebenzoate) to the blending polymeris from about 99.1:0.9 to about 80:20. The poly(alkyleneterephthalate)wherein the alkylene moiety is from 2 to 20 carbons atoms can beselected from the group consisting of poly(ethyleneterephthalate),poly(butyleneterephthalate), etc. up to poly(eicoseneterephthalate). Thepoly(alkyleneterephthalate) is preferably poly(ethyleneterephthalate).The above compositions can exist as blends in the above ranges or ascompositions of poly(p-methylenebenzoate) and poly(m-methylenebenzoate)with interpolymers as defined previously in the same ranges.

Foams obtained by the invented process have high shear sensitivity, slowrate of crystallization and high impact resistance. These propertiescause the foam to be suitable as a structural foam material for highspeed injection molding of thin-wall parts and articles requiringrelatively thick skins and high impact resistance. Typical molded partsand articles of the above description are refrigerator and freezer innerliners, small appliance parts, sporting equipment and equipmenthousings.

The foaming agents most useful in this invention are nitrogen gas,fluorocarbons and certain chemical foaming agents which achieve theireffect by thermal decomposition. The preferred agents are those with aboiling point of from -30° C. to 100° C. under standard conditions.Typical examples of useful organic foaming agents aredichlorodifluoromethane, 1,2-dichloro-1,1,2,2-tetrafluoroethane,monochlorotrifluoromethane, and 5-phenyltetrazole.

Chemical foaming agents which achieve their effect by thermaldecompositions are generally suitable for use in this invention althoughcertain exceptions exist such as azodicarbonamide. Characteristically,the ideal chemical foaming agent decomposes to produce an inert,non-toxic gas, and by-products that are chemically inert. Certainchemical foaming agents such as azodicarbonamide liberate small amountsof ammonia which is known to be aggressive with many condensationpolymers. Similarly certain chemical blowing agents liberate smallamounts of water, which is known to hydrolyze certain condensationpolymers. Further, some of the residual side reaction by-products maycause deterioration in the resin properties. These chemical foamingagents in the course of performing their function generate gases such asnitrogen and carbon dioxide which have extremely low boiling pointsunder normal pressure.

The process of the present invention can be carried out in the presenceof the usual additives generally employed in the production of foamedproducts. These include, for example, cell diameter regulators, foamstabilizers, flame retardants and coloring agents. Such products areutilized in the amounts normally employed with conventional foams.

The following non-limiting examples are given by way of illustrationonly.

EXAMPLE I

One hundred parts by weight of dried poly (p-methylenebenzoate) having anominal I.V. of 0.64 and in the form of 1/8" (nominal) dimensionedpellets were tumble-blended for 15 minutes at room temperature with 0.5parts by weight Celogen AZ, a commercial azodicarbonamide, a product ofUniroyal Chemical, Div. of Uniroyal, Inc., Naugatuck, Conn., 06770, inthe form of a freely-flowing powder. The dry mixture was added to thehopper of a Boy I5S injection molding machine containing a mold thatproduced an edge-gated plaque of dimensions 67 mm by 99 mm by 2.54 mmthick. The molding conditions are those given in column 1 of Table I.With the exception of the last plaque molded, each part experienced atleast a 30-second injection time and a 30-second cure time. For the lastplaque molded, an additional 60 seconds of hold time at melt conditionswere added to simulate typical structural foam time temperatureprocessing conditions. The impact resistance of 100 mil plaques usingGardner drop weight testing system is shown in Table II, third column.The plaque specific gravity was 1.11, or the plaques had a foaming levelof 15%. The cross-section of the plaque was characterized by highdensity skins (nominally) 0.75-1.4 mm in thickness and a low densityfoam core having cells (nominally) 0.05-0.10 mm in dimension uniformlydistributed across the cross-section. The I.V. of the plaque dropped to0.43 after 16,000 (°C.-sec) and 0.42 after 40,000 (°C.-sec).

EXAMPLE II

Example I was repeated, but instead of allowing the plastic to produce afoamed part, the mold was completely filled with plastic undersufficient pressure so that the gas was either vented or redissolved inthe plastic. The processing conditions were identical to those of column1 of Table I. The Gardner drop weight impact data are shown at thebottom of column 2 of Table II. The specific gravity of the plaque was1.31 or nearly equal to the specific gravity of 1.33 for a molded partcontaining no volatile gas. It was thus assumed that theazodicarbonamide, the decomposition by-products and/or the generatedgases caused deterioration in molecular weight.

EXAMPLE III

Example II was repeated, but in a different machine and moldconfiguration. One hundred parts by weight dry poly(p-methylenebenzoate)having a nominal I.V. of 0.64 was tumble-blended for 15 minutes with 0.5parts by weight Celogen AZ (azodicarbonamide) and this mixture was addedto the hopper of a Boy 15/7 injection molding machine containing a moldthat produced a flexural bar 127 mm by 12.7 mm by 3.18 mm thick and atensile bar 184 mm by 19 mm (grip) by 12.7 mm (neck) by 3.18 mm thick.The processing conditions were those shown in column 2 of Table I, andthe mold was again filled completely with plastic. The measured tensileand flexural properties are given in column 2 of Table II. No tensileyield was observed, indicating a brittle polymer, and confirmingazodicarbonamide chemical foaming agent aggressiveness withpoly(p-methylenebenzoate). The I.V. of the molded flexural and tensilebars was 0.45 at 16,000 (°C.-sec) and 0.42 at 40,000 (°C.-sec).

EXAMPLE IV

Example I was repeated, except that 5-phenyltetrazole, a commercialchemical foaming agent, Expandex 5-PT, a product of Olin Corp., ChemicalGroup, Stamford, Conn., 06904, was substituted for Celogen AZ. The finefreely-flowing powder, 5-phenyltetrazole, was added at 0.5 parts byweight. The molding conditions were identical to those given in column 2of Table I. The initial resin I.V. was 0.59. The mold plaques wereimpact tested and the data are shown in Table II, fifth column. Theplaque specific gravity was 1.11 or the plaques had a foaming level of17%. The cross-section appearance was similar to that of Example I. TheI.V. of the plaque was 0.50 after 40,000 (°C.-sec) processing exposure.

EXAMPLE V

Example II was repeated except that 5-phenyltetrazole replacedazodicarbonamide as a chemical foaming agent. The processing conditionswere identical to those of column 1, Table I. The Gardner drop weightimpact data were shown at the bottom of column 4 of Table II. Thespecific gravity of the plaque was 1.33, identical with that of a moldedpart containing no volatile gas.

EXAMPLE VI

Example III was repeated except that 5-phenyltetrazole replacedazodicarbonamide as a chemical blowing agent at the same weight perweight of polymer. The processing conditions were identical to thoseshown in column 2 of Table I. The measured tensile and flexuralproperties are given in column 4 of Table II. The I.V. of the moldedbars was 0.53 at 16,000 (°C.-sec) and 0.47 at 40,000 (°C.-sec).

EXAMPLE VII

Example V was repeated except that poly (p-methylenebenzoate) was moldedwithout a foaming agent, according to processing conditions given incolumn 1 of Table I. The Gardner drop weight impact data are shown atthe bottom of Table II. The I.V. of the molded plaques was 0.53 at16,000 (°C.-sec).

EXAMPLE VIII

Example III was repeated except that the polymer was molded without afoaming agent, according to the processing conditions given in column 2of Table I. The measured tensile and flexural properties are given incolumn 1 of Table II. The I.V. of the molded bars was 0.55 at 16,000(°C.-sec).

                  TABLE I                                                         ______________________________________                                        Molding Conditions for                                                        PPMB and Chemical Foaming Agents                                                           MOLDING CONDITIONS                                                                        Flex, and                                                         Plaques     Tensile Bars                                         ______________________________________                                        Machine        Boy 15S with  Boy 15/7 with                                                   Shutoff Nozzle                                                                              Shutoff Nozzle                                   Front Zone     277° C.                                                                              277° C.                                   Rear Zone      266° C.                                                                              266° C.                                   Injection                                                                     Pressure       400-410 psi   700 (5PT);                                                                    800 (others)                                     Cure Time      30 sec*       30 sec*                                          Injection Time 30 sec        30 sec                                           Screw Speed    100 RPM       100 RPM                                          Injection Speed                                                                              Max           Max                                              Cushion (solid)                                                                              1/8"          1/8"                                             Back Pressure  0             0                                                Feed (solid)   56 mm         64 mm                                            Feed (foam)    47 mm         52 mm                                            Mold Cooling   None          Max                                              Mold Temp (fixed)                                                                            105 ± 5° F.                                                                       88° F.                                    Mold Temp (moving)                                                                           105 ± 5° F.                                                                       89° F.                                    Throat Cooling Max           None                                             ______________________________________                                         *Last shot for each foaming agent was made with 1.5 min residence time at     melt conditions.                                                         

                  TABLE II                                                        ______________________________________                                        PPMB and Chemical Foaming Agents                                                           With AZ    With 5PT                                                         PPMB    Packed         Packed                                                 (only)  Out     Foamed Out   Foamed                                Part Made  TFP     TFP     P      TFP   P                                     ______________________________________                                        Density (Avg.)                                                                           1.33    1.31    1.11   1.33  1.11                                  Foaming Level,                                                                           --      --      15     --    17                                    I.V. of Part                                                                             0.55    0.45    0.43   0.53  0.50                                  Tensile Strength                                                              Yield, psi 7,500   N.D.    --     4.4   --                                    Ultimate, psi                                                                            6,700   6,800   --     6,400 --                                    Elongation                                                                    Yield, %   4.5     N.D.    --     4.4   --                                    Ultimate, %                                                                              375     2.6     --     164   --                                    Tensile Modulus,                                                              Tangent × 10.sup.3 psi                                                             289     328     --     311   --                                    Flex Strength, psi                                                            Yield      11,900  11,700  --     11,700                                                                              --                                    Flex Modulus,                                                                 Tangent × 10.sup.3 psi                                                             322     315     --     314   --                                    100 Mil Plaque                                                                Gardner Drop                                                                  Impact, in-lb                                                                            7,184   2.8     1.4    21    20                                    ______________________________________                                         Notes:                                                                        N.D.--Not determined                                                          TFP--Tensile Bars, Flex Bars, Plaques                                         P--Plaques                                                                    I.V.-- Inherent Viscosity   Note that "injection time" noted in Table I i     the sum of the waiting time between the ejection of the last part and the     initiation of injection for the next part and the actual time of injection     which typically was 1-2 seconds. Typical thermoplastic structural foam     molding operations require more than 60 seconds to cool the molded part.     During this time the next shot of plastic is being held at the melt     temperature. As a result, polymer degradation, as evidenced by a loss in     molecular weight and/or as measured as inherent viscosity (I.V.) can     occur. The poly(p-methylenebenzoate) used in this study had no stabilizer     package with it although it was thoroughly vacuum dried at 120° C.     for up to 24 hours at less than 1 mm Hg vacuum and sealed hot in metal     containers prior to molding. At 16,000 (°C.-sec) hold time, the     0.64 I.V. material, as received, was reduced in I.V. to 0.55, indicating     some deterioration in molecular weight with time at typical thermoplastic     structural foam injection molding conditions.

Very low injection pressures were needed to inject this material,indicating that high speed filling of thin-wall structural foam partscan be achieved.

What is claimed is:
 1. A process for preparing a thermoplastic foamedresin product comprising a polymer composition ofpoly(p-methylenebenzoate) wherein said polymer is foamed in the presenceof a foaming agent selected from the group consisting of an inert,non-toxic gas and a chemical foaming agent which thermally decomposes toproduce an inert, non-toxic gas.
 2. The process of claim 1 wherein saidfoaming agent is present in an amount from about 0.01 to about 10 partsby weight per 100 parts by weight of polymer.
 3. The process of claim 1wherein said foaming agent is selected from the group consisting ofnitrogen gas, carbon dioxide, linear hydrocarbons of from 2 to 8 carbonatoms, halogenated hydrocarbons of from 1 to 6 carbon atoms,dichlorodifluoromethane, 1,2-dichloro, 1,1,2,2-tetrafluoroethane,monochlorotrifluoromethane and 5-phenyltetrazole.
 4. The process ofclaim 1 wherein said foaming agent is 5-phenyltetrazole.
 5. The processof claim 1 wherein said thermoplastic foamed resin product is convertedto an injection molded article.
 6. The process of claim 1 wherein saidpolymer composition comprises poly(p-methylenebenzoate) and a polymerselected from the group consisting of poly(m-methylenebenzoate), apoly(alkyleneterephthalate) wherein the alkylene moiety has from 2 to 20carbon atoms and mixtures thereof, and ratio of saidpoly(p-methylenebenzoate) to said polymer is within the range of fromabout 99.1:0.9 to about 80:20.
 7. The process of claim 6 wherein saidpoly(alkyleneterephthalate) is selected from the group consisting ofpoly(m-methylenebenzoate), poly(ethyleneterephthalate),poly(butyleneterephthalate) and mixtures thereof.
 8. The process ofclaim 6 wherein said polymer composition comprises a physical blend. 9.The process of claim 6 wherein said polymer composition comprises aninterpolymer.